Packaged IC With Solderable Sidewalls

ABSTRACT

A packaged IC wherein a portion of the sidewalls of the packaged IC are solderable metal. A method of forming a packaged IC wherein a portion of the sidewalls of the wire bond pads or the flip chip pads that are exposed by sawing during singulation are solderable metal. A method of forming a packaged IC wherein all of the sidewalls of the wire bond pads or the flip chip pads that are exposed by sawing during singulation are solderable metal and a portion of sidewall of the molding compound is solderable metal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.15/368,413 filed Dec. 2, 2016, and claims the benefit of priority toU.S. Provisional Application 62/262,568, filed Dec. 3, 2015, all ofwhich are incorporated herein by reference in their entirety.

FIELD

Embodiments of the invention are directed, in general, to packaging ofintegrated circuit (IC) chips and, more specifically, forming solderablesidewalls on packaged IC chips.

DESCRIPTION OF THE VIEWS OF THE DRAWINGS

FIGS. 1A and 1B are plan views of lead frames.

FIGS. 2A and 2B are plan views of a lead frame strips.

FIGS. 3A and 3B are cross-sections of IC chips on lead frame stripsprior to singulation.

FIGS. 4A, 4B, 4C, and 4D describe the attachment of a packaged IC to acircuit board by soldering.

FIGS. 5A, 5B, and 5C describe a packaged IC with solderable sidewallsformed according to embodiments.

FIGS. 6A, 6B, and 6C describe a packaged IC with solderable sidewallsformed according to embodiments.

FIGS. 7A, 7B, and 7C describe a packaged IC with solderable sidewallsformed according to embodiments.

FIGS. 8A through 8F are cross sections of the packaged IC in FIG. 6Bdepicted in successive stages of fabrication.

FIGS. 9A through 9E are cross sections of the packaged IC in FIG. 7Bdepicted in successive stages of fabrication.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Embodiments of the disclosure are described with reference to theattached figures. The figures are not drawn to scale and they areprovided merely to illustrate the disclosure. Several aspects of theembodiments are described below with reference to example applicationsfor illustration. It should be understood that numerous specificdetails, relationships, and methods are set forth to provide anunderstanding of the disclosure. One skilled in the relevant art,however, will readily recognize that the disclosure can be practicedwithout one or more of the specific details or with other methods. Inother instances, well-known structures or operations are not shown indetail to avoid obscuring the disclosure. The embodiments are notlimited by the illustrated ordering of acts or events, as some acts mayoccur in different orders and/or concurrently with other acts or events.Furthermore, not all illustrated acts or events are required toimplement a methodology in accordance with the present disclosure.

For the purposes of this description, the term “lead frame strip” isunderstood to refer to a plurality of lead frames (FIG. 1A) coupledtogether by horizontal 202 and vertical 204 saw streets (FIG. 2A).

The term “packaged IC” is understood to refer to an IC chip attached toa lead frame and encapsulated with molding compound.

The term “solderable metal” is understood to refer to metals whichsolder wets readily. Examples include silver, gold, nickel, palladium,tin, solder and alloys thereof and an ink or paste comprised of a matrixcontaining solderable metal particles, The matrix may be a material suchas a polyimide or epoxy resin or a solder flux.

Packaged ICs such as Small Outline No-Lead (SON) and Quad Flat No-Lead(QFN) ICs are typically fabricated by first attaching IC chips to ametal lead frame strip, encapsulating them with molding compound, andthen singulating the encapsulated IC chips by sawing them apart alongsaw streets to form individual packaged ICs.

The lead frame strip 200 (FIG. 2A) for a packaged wire bonded IC 300(FIG. 3A) is typically laid out to include for each wire bond lead frame100 (FIG. 1A) an IC chip pad 102 and coordinated wire bond pads 104 alsoreferred to as lead frame pads. The lead frame strip 210 (FIG. 2B) for apackaged flip chip IC 301 (FIG. 3B) is typically laid out to include foreach flip chip lead frame 110 (FIG. 1B) coordinated flip chip pads 106also referred to as lead frame pads.

In FIGS. 2A and 2B, lead frame strips, 200 and 210, are commonly formedby connecting multiple lead frames 100 or 110 together with horizontal202 and vertical 204 saw street metal strips. These saw street metalstrips, 202 and 204 are later removed by sawing (singulation) to produceindividual packaged ICs.

Lead frame strips 200 and 210 are typically made of a base metal such ascopper or a copper alloy. The lead frame strips, 200 and 210, may beplated with layers of solderable metal, such as a layer of nickelfollowed by a layer of palladium to prevent oxidation of the base metalto facilitate soldering.

A cross section of a lead frame strip 200 with attached IC chips 304 isillustrated in FIG. 3A. IC chips 304 are mounted on the IC chip pads 102with a conductive epoxy or solder 307. In addition, IC chips 304 areelectrically connected to the wire bond pads 104 with wire bonds 305.Wire bond pads 104 from adjacent lead frames 100 are attached toopposite sides of a saw street metal strip 204. The mounted IC chips304, IC chip pads 102, wire bond pads 104 and saw street metal strips204 are encapsulated with molding compound 308. The bottom surface 316of the packaged wire bond IC strip 300 typically is covered with abottom solderable metal 312 that remains exposed to enable individualpackaged wire bond ICs to be soldered to a circuit board. Individualpackaged wire bond ICs 400 (FIG. 4A) may then be singulated by sawingthrough the saw street metal strip 204 and sawing through the moldingcompound 308 in the saw street 205.

A cross section of a lead frame strip 210 with IC flip chips 306 isillustrated in FIG. 3B. IC flip chips 306 are integrated circuit chipsthat are flipped upside down and soldered to flip chip pads 106,typically using solder joints 307. Flip chip pads 106 from adjacent leadframes 110 are connected to opposite sides of a saw street metal strip204. The IC flip chips 306, flip chip pads 106, and saw street metalstrips 204 are encapsulated with molding compound 308 to form thepackaged flip chip IC strip 301. The bottom surface 316 of the packagedflip chip IC strip 301 is typically covered with bottom solderable metal314 that remains exposed to enable individual packaged flip chip ICs tobe soldered to a circuit board. Individual flip chip ICs 401 (FIG. 4B)may then be singulated by sawing through the saw street metal strip 204and sawing through the molding compound 308 in the saw street 205.

An individual packaged wire bond IC 400 after singulation is shown inFIG. 4A. The sidewall 410 of the wire bond pad 104 is exposed during thesingulation process. A metal oxide 408 may form the sidewall 410 whenexposed to air. This metal oxide 408 may prevent solder from wetting thesidewall 410 and forming a reliable solder joint 406.

A circuit board 402 with solder paste 405 on circuit board leads 404 isshown in FIG. 4B. In FIG. 4C, a packaged wire bond IC 400 is attached tothe circuit board 402 with solder joints 406 between the circuit boardleads 404 and the bottom solderable material 312 covering the bottom ofthe wire bond pads 104 and the IC chip pads 102.

As shown in FIG. 4C, if the oxidized metal 408 is adequately removedfrom the sidewall 410 prior to soldering then, a reliable solder joint406 may be formed where the solder wets the sidewall 410 of the wirebond pad 104.

If the oxidized metal 408 is not adequately removed from the sidewall410 of the wire bond pads 104 prior to soldering than an unreliablesolder joint 407 may be formed. As is shown in FIG. 4D, the solder doesnot wet the sidewall 410 when the oxidized metal 408 is present. Anunreliable solder joint 407 may lead to delamination of the packagedwire bond IC 400 from the circuit board 402.

FIG. 5B shows a packaged wire bond IC 500 with solderable metal 502(metal that solder readily wets) on the sidewall of the wire bond pad104 that is exposed during singulation. The embodiment is illustratedusing a packaged wire bonded IC 500 but generally applies to allpackaged ICs that are formed using lead frame strips. For example, anembodiment packaged flip chip IC with solderable metal 502 on thesidewall of the flip chip pad 106 may also be used for illustration. Thesolderable metal 502 may be a metal such as silver, gold, nickel,palladium, tin, solder or an alloy such as AgSn.

FIG. 5A shows a cross section of a lead frame strip 200 with packagedwire bond ICs 500 prior to singulation by sawing through the saw street504. In this example, the saw street metal strip 204 (FIG. 3A) plus aportion of the wire bond pads 104 attached to opposite sides of the sawstreet metal strip 204 are replaced with solderable metal 502.

As shown in FIG. 5B the packaged wire bond IC 500 may be singulated bysawing through the saw street 504. During singulation the solderablemetal 502 on the sidewalls of the wire bond pads 104 is exposed. Asshown in FIG. 5C, solder wets this solderable metal 502 forming a strongsolder joint 406 between the solderable metal 502, the bottom solderablemetal 312 covering the bottom of the wire bond pad 104, and the lead 404on the circuit board 402.

FIG. 6B shows a packaged wire bond IC 600 with a first solderable metal606 on the sidewall of the wire bond pad 104 that is exposed duringsingulation and also with a second solderable metal 608 on a portion ofthe sidewall of the molding compound 308 that is exposed duringsingulation.

FIG. 6A shows a cross section of a lead frame strip 200 with packagedwire bond ICs 600 prior to singulation by sawing through the saw street604. In this example, the saw street metal strip 204 (FIG. 3A) plus aportion of the wire bond pads 104 attached to opposite sides of the sawstreet metal strip 204 is replaced with a first solderable metal 606forming a first solderable sidewall. In addition, a portion of themolding compound 308 in the saw street 604 and a portion of the moldingcompound on opposite sides of the saw street 604 are replaced with asecond solderable metal 608 forming a second solderable sidewall.

After singulation by sawing, as shown in FIG. 6B, the exposed sidewallof the packaged wire bond IC 600 is comprised primarily of first andsecond solderable metals, 606 and 608. These solderable metals, 606 and608, readily wet and form strong solder joints 406 when the packagedwire bond IC 600 is attached to a circuit board 402 as shown in FIG. 6C.The first solderable sidewall formed of the first solder metal 606 plusthe second solderable sidewall formed of the second solderable metal 608provides an increased area for the solder to wet thus facilitating theformation of a larger and more robust solder joint 406 as is illustratedin FIG. 6C.

For illustration two different solderable metals, 606 and 608 are usedto form a sidewall on a portion of the wire bond pad 104 and to form asidewall on a portion of the molding compound 308. Alternatively, thefirst and second solderable metals, 606 and 608, may be the samesolderable metal. One solderable metal that adheres to both the wirebond pad 104 and to the molding compound 308 may be used. An example inkor solderable screen print paste may have solderable metal particlessuspended in a polyimide or epoxy resin or in a solder flux.

FIGS. 7A-7C illustrate an example where greater than about 0% and lessthan about 100% of the sidewall of the wire bond pad 104 that is exposedduring singulation is covered with solderable metal 702. Unlike in FIGS.5A and 6A where the wire bond pads are completely sawed through andreplaced with a solderable metal; in this embodiment the saw streetmetal strip 204 and the wire bond pads 104 on opposite sides of the sawstreet metal strip 204 are partially sawed through and replaced with asolderable metal 702. Therefore, a partial saw street metal strip 704remains between the wire bond pads 104. The partial saw street metalstrip 704 preserves the adhesive bond between the molding compound 308,the solderable metal 702, and the wire bond pads 104 duringencapsulation. The partial saw street metal strip 704 may also providereinforcement that may reduce warpage during handing.

A method for forming the embodiment packaged wire bond IC 600 shown inFIG. 6C is described in the cross sections illustrating the majorprocessing steps in FIGS. 8A through 8F. The embodiment is illustratedwith a packaged wire bonded IC 600 but it generally applies to otherpackaged ICs such as packaged flip chip ICs.

FIG. 8A shows two packaged wire bond is 600. IC chips 304 are mounted onIC chip pads 102 in lead frame strip 200. Wire bonds 305 electricallyconnect the IC chips 304 to wire bond pads 104 also in the lead framestrip 200. Wire bond pads 104 from two adjacent lead frames 100 (FIG.1A) are attached to opposite sides of the saw street metal strip 204.The IC chips 304, the wire bonds 305, the IC chip pads 102, the wirebond pads 104, and the saw street metal strips 204 are encapsulated inmolding compound 308.

In FIG. 8B a trench 805 is cut through the saw street metal strip 204and cut through portion of the wire bond pads 104 attached on oppositesides of the saw street metal strip 204. The trench 805 may also be cutthrough a portion of the molding compound 308. The trench 805 may be sawcut or laser cut for example. The trench 805 may be at least 0.01 mm perside wider than the saw street 804. In an example embodiment the trench805 is 0.03 mm per side wider than the saw street 804.

In FIG. 8C-1 the trench 805 is partially filled with a second solderableink 810 using an ink jet printer 806. The second solderable ink 810approximately fills the portion of the trench 805 cut into the moldingcompound 308. The second solderable ink 810 may contain solderable metalparticles dispersed in a polyimide or epoxy resin or a solder flux, forexample. The solderable metal particles may be a metal or metal alloysuch as silver, silver-tin, solder, gold, nickel, platinum, palladiumand alloys thereof.

Alternatively as shown in FIG. 8C-2, a screen printing mask 815 may beused to screen print a second solderable screen print paste 811 toapproximately fill the portion of the trench 805 cut into the moldingcompound 308. The second solderable screen print paste 811 may be aconductive solder paste designed to adhere to molding compound 308.

In FIG. 8D-1 the remainder of the trench 805 is approximately filledwith a first solderable ink 812 using an ink jet printer 806. The firstsolderable ink 812 may be a polyimide or epoxy resin containingsolderable metal particles, may be an ink composed of powdered soldersuspended in a flux, or it may be an ink composed of powdered solderplus solderable metal particles suspended in a flux. The solderablemetal particles may be a metal or metal alloy such as silver,silver-tin, solder, gold, nickel, platinum, palladium and alloysthereof.

As shown in FIG. 8D-2, screen printing mask 815 may be used to screenprint a first solderable screen print paste 813 to approximately fillthe remainder of the trench 805.

Referring now to FIG. 8E, the second and first solderable inks 810 and812 or the second and first solderable screen print pastes, 811 and 813,may be annealed to evaporate solvent and to cause the inks or pastes toform first and second solderable metals, 606 and 608. The annealtemperature may be in the range of about 80° C. to 300° C., for example.

FIG. 8F shows the packaged wire bond ICs 800 after singulation bycutting through the saw street 804, The packaged wire bond ICs 800 havea first solderable sidewall composed of solderable metal 606 on thesidewalls of the wire bond pads 104 and second solderable metal 608 on aportion of the sidewalls of the molding compound 308.

The depth of the trench 805 may be varied as desired. The trench 805 maypenetrate little into the molding compound 308, may penetrate through amajority of the molding compound 308, or may penetrate completelythrough the molding compound 308. Alternatively, one solderable ink orsolderable screen print paste that adheres to both molding compound 308and to wire bond pads 104 may be used to fill the trench 805 using aone-step fill operation.

With this embodiment when the packaged wire bond ICs 800 are singulated,no lead frame base metal of wire bond pads 104 is exposed. Asillustrated in FIG. 6B, using this embodiment, the increased sidewallarea that is provided by solderable metals 606 and 608 enables a larger,more robust solder joint 406 to be formed during attachment of thepackaged wire bond IC 600 to a circuit board 402.

A method for forming the second embodiment packaged wire bond IC 700shown in FIG. 7C, is described in the cross sections illustrating themajor processing steps in FIGS. 9A through 9E. The embodiment isillustrated with a packaged wire bonded IC 700 but generally applies toall packaged ICs including packaged flip chip ICs.

FIG. 9A shows two packaged wire bond ICs 700 prior to singulation bysawing through saw street 904. IC chips 304 are mounted on lead frame ICchip pads 102 in lead frame strip 200. Wire bonds 305 electricallyconnect the IC chips 304 to wire bond pads 104 also in the lead framestrip 200. Wire bond pads 104 from two adjacent lead frames 100 (FIG.1A) are attached to opposite sides of the saw street metal strip 204.The IC chips 304, the wire bonds 305, the IC chip pads 102, the wirebond pads 104, and the saw street metal strips 204 are encapsulated inmolding compound 308.

In FIG. 9B a trench 905 is cut part way through saw street metal strip204 and part way through a portion of the wire bond pads 104 that areattached to opposite sides of the saw street metal strip 204. A partialsaw street metal strip 704 remains between the two wire bond pads 104.The trench 905 may be cut by sawing or by cutting with a laser. Thetrench 905 may be at least 0.01 mm per side wider than the saw street904. In an example embodiment the trench 905 is 0.03 mm per side widerthan the saw street 904.

The trench 905 may be cut to a depth of between greater than about 0%and less than about 100% of the way through the saw street metal strip204 and through a portion of the wire bond pads 104. In an exampleembodiment the trench 905 is cut through about 80% of the saw streetmetal strip 204.

In FIG. 9C-1 the trench 905 is filled with a solderable ink 705 using anink jet printer 904. The solderable ink 705 may be a solder paste or asolvent such as a polyimide or epoxy resin containing solderableparticles comprised of a solderable metal or metal alloy such as silver,silver-tin, solder, gold, nickel, platinum, tin, palladium and alloysthereof.

Alternatively as shown in FIG. 9C-2, the trench 905 may be filled with asolderable screen print paste 707 using a screen printing mask 915.

Referring now to FIG. 9D, the solderable ink 705 or solderable screenprint paste 707 may be annealed to evaporate solvent and to cause theink or solderable screen print paste to cure and reflow into asolderable metal 702 filling the trench 905. The thermal treatment maybe in the range of about 80° C. to 300° C., for example.

FIG. 9E shows two packaged wire bond ICs 700 after completingsingulation by cutting through saw street 904. The packaged wire bondIC's 700 have solderable metal 702 formed on a portion of the sidewallsof the bond pads 144

With this embodiment, between greater than about 0% and less than about100% of the sidewall on the wire bond pad 104 that is exposed duringsingulation is composed of a solderable metal 702. As illustrated inFIG. 7B, using this embodiment, strong solder joints 406 may formed tothe solderable metal 702 during the soldering of the packaged wire bondIC 700 to circuit board 402.

While various embodiments of the present disclosure have been describedabove, it should be understood that they have been presented by way ofexample only and not limitation. Numerous changes to the disclosedembodiments can be made in accordance with the disclosure herein withoutdeparting from the spirit or scope of the disclosure. Thus, the breadthand scope of the present disclosure should not be limited by any of theabove described embodiments. Rather, the scope of the disclosure shouldbe defined in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A packaged IC comprising: an IC chip; an IC chippad coupled to the IC chip; a wire bond pad or a flip chip pad coupledto the IC chip, the wire bond pad or flip chip pad having an exposedsidewall; molding compound encapsulating the IC chip, a portion of theIC chip pad, and a portion of the wire bond pad, the molding compoundhaving a sidewall; and a first solderable metal extending from a bottomsurface of the packaged IC along a sidewall of the packaged IC.
 2. Thepackaged IC chip of claim 1 wherein greater than about 0% and less thanabout 100% of the sidewall of the wire bond pad or the flip chip pad isexposed.
 3. The packaged IC chip of claim 1, wherein 0% of the sidewallof the wire bond pad or the flip chip pad exposed.
 4. The packaged ICchip of claim 3 wherein greater than about 0% and less than about 100%of the sidewall of the molding compound is exposed.
 5. The packaged ICchip of claim 1, further comprising a second solderable metal extendingfrom the first solderable metal along the sidewall of the packaged IC.6. The packaged IC of claim 5, wherein the first solderable metal andthe second solderable metal are different metals.
 7. The packaged IC ofclaim 4, wherein the first solderable metal and the second solderablemetal are the same metal.
 8. The packaged IC of claim 1, wherein thefirst solderable metal is selected from the group consisting of: silver,an alloy of silver, an alloy of silver and tin, an alloy of silver andtin and copper, gold, an alloy of gold and tin, palladium nickel analloy of nickel and palladium, an alloy of nickel and palladium, andgold, solder, and platinum.